DIFFERENTIATION AND DEVELOPMENT 87
Perhaps these differences are not surprising, given that
wall composition and wall bonding are intimately
linked with cell shape (Chapter 4). However, the lack
of consistency between the fungi suggests that wall
composition alone cannot provide a common basis for
understanding dimorphism.Differences in cellular signaling and regulatory factors
Environmental signals often affect cellular behavior
through a signal transduction pathway leading to
altered metabolism or gene expression. The intra-
cellular factors involved in this include calcium and
calcium-binding proteins, pH, cyclic AMP, and protein
phosphorylation mediated by protein kinases.
The complexing of calcium with the calcium-
binding protein calmodulin was found to be essential
for mycelial growth of Ophiostoma ulmi(Dutch elm dis-
ease); otherwise, the fungus grew in a yeast form.
Consistent with this, the levels of calmodulin typically
are low (0.02–0.89μgg−^1 protein, for 14 species)
in fungi that always grow as yeasts, but tend to be
higher (2.0 –6.5μgg−^1 protein) in mycelial fungi
(Muthukumar et al. 1987). An external supply of
calcium is required for apical growth of Neurospora
crassaand many other fungi, and calcium is needed in
larger amounts for initiation of the M form than for
budding in C. albicans. High intracellular levels of
cyclic AMP are associated with yeast growth in Mucor
species, C. albicans, H. capsulatum, B. dermatitidis, and
P. brasiliensis, whereas low cAMP levels are associated
with hyphal growth. The supply of cAMP externally can
also cause a dimorphic switch. Changes in cytosolic
pH have been associated with the M–Y transition of C.
albicans, and with polar outgrowths in some other cell
types. Thus, it seems clear that intracellular signaling
compounds are associated with phase transitions, but
they are the messengers and mediators not necessarily
the direct cause of changes in cell morphology.
Differences in gene expression
Differences in gene expression in the M and Y phases
can be detected by extracting messenger RNA and
comparing the mRNA banding patterns by gel elec-
trophoresis. Or, preferably, by using the messenger
RNAs as templates to produce complementary DNA
(cDNA), which is more stable. An example of this
approach, though relating to the development of
fungal fruitbodies, is discussed later in this chapter. In
several cases it has been shown that a few polypeptides
are constantly associated with only the M or the Y
phase. But for dimorphic fungi there seems to be no
case in which a gene or gene product is obligatorily
involved in the generation of cell shape. Harold
(1990), in a review of the control of cell shape in
general, wrote: “... form does not appear to be hard-
wired into the genome in some explicit, recognizable
fashion. It seems to arise epigenetically... from the
chemical and physical processes of cellular physiology.”
In other words, there are no cell-shape genes, as such!A potentially unifying theme – the vesicle
supply center
Given that a wide range of environmental factors can
influence cell shape, although in different ways in dif-
ferent organisms, Bartnicki-Garcia and his colleagues
(see Bartnicki-Garcia et al. 1995) have adopted a dif-
ferent approach, using computer simulations as a basis
for understanding the dimorphic switch and fungal
morphogenesis in general. The central feature of
the simulations is a postulated vesicle supply center
(VSC) such as a Spitzenkörper, envisaged as releasing
vesicles in all directions to “bombard” the cell mem-
brane and synthesize the wall. As shown in Fig. 5.1,
it is then possible to simulate almost any change in
growth form by changing the spatial location and/or
the rate of movement of the VSC. If the VSC remains
fixed then the cell will expand uniformly as a sphere.
If the VSC moves rapidly and continuously forward it
will produce an elongated structure, like a hypha. If
it moves more slowly it will produce an ellipsoidal
cell; if it moves to one side of the hypha it will cause
bending, and so on. Thus, the key to morphogenesis
could be the rate of displacement of the VSC. Other
variations in shape and size could occur if the rate of
vesicle release from the VSC were altered relative to
its rate of movement. We saw in Chapters 3 and 4
that the apical vesicle cluster is intimately associated
with cytoskeletal components that are implicated in
cellular movements, so it is feasible that the rate of
displacement of a VSC could be altered by factors that
affect the cytoskeleton. Video-enhanced microscopy
of hyphal tips shows that the Spitzenkörper exhibits
random oscillations in growing hyphal tips, associated
with oscillations in the direction of growth, and the
Spitzenkörper also can divide to leave a “daughter” VSC
at a future branch point (Chapter 3).
We return to the subject of dimorphism in Chapter
16, when we discuss the human-pathogenic fungi.Infection structures of plant pathogensFungal pathogens of plants can penetrate either
through an intact host surface or through natural
openings such as stomata, but in any case the inva-
sion of a host is preceded by production of specialized
infection structures of various types (Fig. 5.2). In
this section we focus on plant-pathogenic fungi,
but equivalent structures are produced by fungal